Fire detection apparatus



Oct. 26, 1954 A. a. B. METCALF 2,692,982

FIRE DETECTION APPARATUS Filed April 19, 1948 JAW- Irlvnrflur ARTHUR 6.. B. METGALF Patented Oct. 26, 1954 vNlTED .STATZ S PATENT OFFICE "FIRE'DETEOTION APPARATUS Arthur G. .B..Metcalf, Milton, Mass.

. Application April 19, 1948, Serial No. 22,028

(Cl. sea-227) 3 Claims. 1

This invention relates to fire: detection and prevention in vehicles, particularly in aircraft;

The airplane is not only inherently-subject to fire hazards not common to other vehicles, but, in addition, peculiarly vulnerable to fire. The very factors which promote flying efficiency, for example, lightness of structure, reduction in the weight and frontal area of the power plant, the use of light metals, which have poor heat resistance, and improvement in the volatility of fuels, necessarily increase the danger of fire. High altitude flying has introduced new hazards, such as arcing in electrical systems, and inadequacy of cooling in air-cooled engines, and has given rise to the requirement for'supercharging and oxygen supply systems which are in themselves potential sources of fire.

The modern airplane is a complex assembly often powered by several engines, and equipped with a vast number of intricate electrical, hydraulic and mechanical devices'for performing the various indicating, control,.and communicaticn functionsnecessary for operation and navigation of the craft. stringent spacerequirements dictate that large quantities of high octanegasoline he carried in wing tanks, in proximity to the engine nacelles. While. considerable research and effort has been directed towardminimizing-fire hazards in the design and arrangement of electrical and hydraulic systems and toward the development of fireproof fuel tanks, it isinevitable that increasing complexity of any mechanism which utilizes highly combustiblefuels should result in an increasing probability of fire. The hazard is aggravated inthe case'of the military airplane by the carrying of explosives and exposure toenemy gunfire.

As a result of extensive investigation by-mil;- tary and civilian authorities, both in this-country and abroad, it hasbeen determined that the fires most disastrous and difiicult to control, that is, fires occurring in flight, usually start in the engine, and that such fires can be extinguished in flight without serious damage to the airplane if the pilot is warned of the fire and applies the proper fire-fighting measures in time. The need for quick action and especially for quick detection has been dramatically illustrated in several known instances where fire in an engine nacelle has caused wing failure in less than a minute after the fire started and before the pilot was aware that his engine was on fire. In short, one of the major conclusions to be drawn from such investigations is that the virtual elimination of serious accidents due to fire in flight, with the resulting incalculable saving in lives and property, can be achieved, and at presentirequires only the development of a reliable alarm system capable of instantly detecting theoutbreak of fire. In such a device, reliability is as important as quick action. The usual fire fightingproce- 2. dures involve shutting down the engine, which may necessitate a forced landing. It can be readily appreciated that one or two such experiences as a result of false alarms may lead pilots to disregard the fire warning system.

Erqaeriments have been madewith a variety of fire alarm devices and several types are currently used on aircraft. .The detecting elements may be generally classified as thermocouples, thermal expansion switches, and fusible elements. The fundamental disadvantage of such. elements for fire detection purposes is that theyaretemperature-responsive, rather than flame-responsive and therefore subject to an inherent time delay which. may vary considerably, depending on the distance of the elementfrom the flame andon local condi tions affecting heat transmission. To ensure reasonably quick operation, temperature-responsive elements mustbe placed in the engine so .asto be downwind from and fairly close to the possible sources of fire, and, for this reason, the elements themselvesare usually destroyedwhen fire occurs. Care must be taken, also, toensure that detecting elements are not so located as to be cooled by air currents, as, under such conditions, these devices have failed to give warning of a serious fire burning only a short, distance away. This .diificulty can be minimized only by exhaustive. experimentation witheach new type of engine installation todetermine the proper location of detecting element's, and the use of a large, number of detectingelements. The thermocouple. types of detectors and some of the thermal expansion types require frequent, delicate adjustment and must be used with sensitive circuitswhichare unstable in operation. Being essentially. mechanical switch ing devices,- they are susceptible to vibration and acceleration. The fusibletype is additionally objectionable because it is destroyed on operation and must bereplaced, and does notpermit. checking of the system. In order to provideanything approaching complete fire protection, a large number of suchelemen-ts, spaced not more than a few inches apar.t,would.be required allaround each engine. Analternative, the continuous strip element. ofthefusible type, .is. now considered by aircraft authorities to be obsoletebecause of many inherent drawbacks in itsuse. Since. the weight of the elementsthemselves and. the wiring for such complete protectionwould .be prohibitive, the present fire detection systems on aircraftnecessarily, represent a compromise between weight and safety considerations.

Furthermore, the development .of jet propulsion for-aircraft has givenrisetoa fire detection problem for which the thermally operatedsystems are Wholly inadequate. In. comparison to a recipro eating engine of comparable power, thepotential .dangerarea of a jet engine is considerably greater, and the normal ambient temperature in the-enginenacelle is; considerably higher. Moreover,

excessive cooling of the jet pipe is to be avoided, as efficiency is dependent on maintaining a jet temperature as high as the internal metal parts will withstand. Insulation between the engine and the surrounding structure is ordinarily provided only by a relatively narrow air-gap between the engine itself and a conical metal shroud. It is apparent that, with such an arrangement, the occurrence of fire, upon failure of a high pressure fuel line or turbine blade, for example, is likely to be so rapid and localized that even very closely spaced thermal detectors would fail to respond in time to prevent severe damage to the airplane. Difiiculty from false alarms are also encountered because of the rapid temperature fluctuations and the small differential between normal operating and excessive temperature.

It is accordingly the object of this invention to provide a fire detection system, especially suitable for aircraft, which, although light in weight as compared to systems now in use for the purpose, is reliable and stable in operation, which responds instantly, which affords complete coverage of the danger area on all types of airplane engines, ineludin many locations inaccessible for thermal detectors, which is substantially unaffected by local temperatures and air currents, which responds to fiame at a considerable distance from the detecting elements, so that the elements need not be located in the flame, which is impervious to vibration and acceleration effects, which is capable of performing functions beyond the scope of many of the present devices, and which can be readily adapted to any engine installation.

This invention comprises generally an element, which undergoes an electrical change when subjected to radiation from a fiame, connected to a suitable amplifier, the output current of which actuates the desired control devices. Between the detecting element and the flame may be interposed a filter, designed to pass a selected band of frequencies and to filter out extraneous radiation.

The many advantages and novel features of this device are more fully explained in the following description with reference to the accompanying drawings, of which:

Fig. 1 is a diagrammatic side view of a typical aircraft engine with the fire alarm device installed; and

Fig. 2 is a diagrammatic side view of a turbojet engine (the turbine section broken away for convenience), with the fire alarm device installed to supervise the combustion chamber and jet pipe area.

According to this invention, a detecting element sensitive to the frequencies radiated by flame is mounted in the engine nacelle in a suit able location so as to receive direct radiation from a flame occurring within the space to be supervised. The detecting element is connected to an electronic amplifier of any of the suitable types known in the art, so that the change in conductivity of the element under the influence of radiation is amplified, and the resulting current may be used to operate a control or alarm device. Examples of suitable detecting elements include present commercial types of caesium-oxide-onsilver photocells, bolometers, and selenium cells, which have good sensitivity in the visible red and infrared region, lead sulphide semi-conductors which have sensitivity extending well into the infrared, and special types of sodium and other photocells which are sensitive to ultra violet. If the cell is to be mounted in a location where it is subject to extraneous light, it is preferably enclosed in a housing provided with an optical filter,

which has good transmission in a selected nonvisible region but cuts out part or all of the visible spectrum. The frequency band selected for transmission should correspond, wherever possible, to the region of peak sensitivity of the detecting element. It has been found that a caesium oxide photocell or lead sulphide semi-conductor used in conjunction with a filter which passes infrared, but shuts out substantially all the visible spectrum, will detect a small fiame at considerable distance against the normal daylight background. Either type of cell responds instantaneously to the flame radiation, the sensitivity of the fiame detector system being determined by the degree of amplification.

In an aircraft radial engine installation as illustrated in Fig. 1, the space occupied by the cylinders is ordinarily referred to as the power zone, and that occupied by the fuel pump, oil pump and other engine accessories, as the accessory zone. The two zones, here generally indicated by the numerals 4 and 5, are divided by a fire wall 6, which serves to retard the spread of fire from one zone to another. A reasonably small fire in the power zone can ordinarily be blown out when detected, by proper measures such as stopping the engine, thus stopping the supply of fuel which might feed the fire, feathering the propeller, and opening the cowling, if adjustable, to increase ventilation. Prompt detection is imperative, however, if the fire is to be successively extinguished, before great damage is done to the engine and surrounding structure. A fire originating in the accessory zone, if promptly detected, can be readily extinguished by a standard aircraft type of chemical extinguisher.

According to Fig. 1, fire detection in the power zone is provided by a photocell 8 mounted approximately in the plane of the cylinders H and facing slightly inward and toward the rear of the cowling l. The field of vision of the photocell cathode is preferably so chosen as to exclude direct rays of the sun which might enter the rear of the cowling in certain attitudes of flight. Since fiame occurring in the power zone will be blown toward the rear, this disposition of the photocell is suitable for covering one quadrant of the space in this zone. Three other photocells 9, H) and another, not shown, are similarly mounted at intervals around the engine and connected to amplifier l2. Radiation impinging on any of the photocells from a source within its optical field lowers the impedance of the cell. This change in conductivity is amplified and the resulting current output from amplifier I2 is used to operate a signal lamp l3 on the instrument panel of the airplane. It is, of course, understood that the output current of amplifier l2 may operate any other electrical control or indicating device. Since zone 4 is ordinarily open to daylight through the rear edge of the cowling, the photocells in this area are preferably enclosed and provided with optical filters, schematically indicated by I! which shut out the visible spectrum. According to wellknown methods, the detecting photocells may be connected either in parallel or to separate amplification channels feeding into a common output circuit.

In zone 5, two photocells l4 and i5 placed on opposite sides near the outer edge of the accessory compartment are ordinarily adequate to supervise the entire region. These photocells are connected to amplifier l6 which actuates a signal light l3 or other control or indicating device.

It will be noted that in both the power zone and accessory zone installations, the proper location of detecting elements can be simply determined by optical alignment, that is, full coverage of a zone requires only that the parts of the area blocked from the view of one photocell be within the view of another. Thus, installation of this device on a new type of engine presents no bstacle comparable to the lengthy experimental determinations of local temperatures and air currents required for effective use of the thermally operated fire alarm systems hitherto used.

In Fig. 2, two photocells l8 and I9 are shown mounted in the air space 20 between jet engine 2| and the shroud 22. Flame occurring anywhere inside the shroud will illuminate at least one of the photocells. The resulting change in conductivity of the cell is amplified by amplifier 23 and the output current of amplifier 23 operates a signal or control device which may, for example, be a signal lamp 24 mounted on the instrument panel of the airplane. As for the reciprocating engine installation of Fig. 1, the proper location of the detecting elements is simply determined by optical considerations. An optical filter may be used where necessary to shut out extraneous visible light.

Any of the systems shown gives a fire out indication, that is, the signal light goes out, when the photocell returns to its dark, high impedance, condition. Tests have established that these detectors, if properly installed, are able to survive successive fires without failure even in cases where the engine or other structure has failed under repeated fire tests. A false fire out indication cannot occur, however, as it may with thermal detectors when the fire moves away from the detecting elements, because a photocell will be subject to radiation as long as any flame exists in the supervised space. Furthermore, this device operates instantly upon cessation of flame, whereas a thermal element will operate only after a considerable and unpredictable delay because of persisting high tempera ture in the surrounding parts. The photoelectric detector is ready to operate as before if fire i;

should again break out. This repeat action is important because, under emergency conditions, the pilot may be forced to start the engine again after the fire is extinguished, and there is the possibility of recurrence of fire in the damaged engine.

The advantages of this system in both illustrations will be at once apparent. The alarm operates immediately upon outbreak of fire, so that the pilot has a maximum time to apply fire fighting measures before serious damage occurs. Furthermore, since the detecting element is excited by radiated rather than conducted heat, false alarms due to ambient temperature fluctuations, and failures due to cooling by local air currents, are eliminated. The small number of elements required to supervise a given volume represent a considerable saving in weight, for example, in Fig. 1, two photocells l4 and I provide more complete protection in the accessory zone, where unpredictable fire draft conditions make the problem of locating thermal elements particularly difficult, than was formerly obtainable with from ten to fifteen thermal elements. Another advantage is the ease with which the system may be checked. The condition of some types of thermal elements can be checked only roughly, and the fusible type cannot be checked at all. This system can be operated for inspection purposes by exposing the cells to light from a small incandescant lamp, for example, a test lamp permanently installed inside the nacelle. A further advantage is the insensitivity of the detecting elements to mechanical vibration and acceleration.

Since certain changes may be made in the above-mentioned article and different embodiments of the invention are possible without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not in a limiting sense.

What is claimed is:

1. A fire detection system for aircraft adapted to detect fire in aircraft engine nacelles, including a photoelectric detecting element adapted to undergo an electrical change upon exposure to infrared frequencies radiated by a flame, means for mounting said element in a fixed position to supervise a selected space within its optical field, optical filter means adapted to transmit said infrared frequencies to said element and reject substantially all visible radiations, means for amplifying said electrical change, and means actuated by said amplifying means for effecting a response upon occurrence of said electrical change.

2. A fire detection system for aircraft adapted to detect fire in aircraft engine nacelles, including a plurality of photoelectric detecting elements adapted to undergo an electrical change upon exposure to infrared frequencies radiated by a flame, means for mounting said elements in fixed positions to supervise a selected space, optical filter means adapted to transmit said infrared frequencies to said elements and reject substantially all visible radiations, means for amplifying said electrical change, and means actuated bysaid amplifying means for effecting a response upon occurrence of said electrical change.

3. A fire detection system for aircraft adapted to detect fire in aircraft engine nacelles, including a plurality of photoelectric detecting elements adapted to undergo an electrical change upon exposure to infrared frequencies radiated by flame, optical filter means adapted to transmit said infrared frequencies to said elements and reject substantially all visible radiations, means for mounting and aligning said elements in fixed positions within an aircraft engine nacelle about the circumference of said nacelle to supervise substantially the entire nacelle so that flame occurring within said nacelle will illuminate at least one of said detecting elements, means for amplifying said electrical change, and means actuated by said amplifyin means for efiecting a response upon occurrence of said electrical change.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,959,702 Barker May 22, 1934 2,177,493 Koulichkov Oct. 24, 1939 2,376,920 Jones May 29, 1945 2,385,976 Evans et a1. Oct. 2, 1945 2,455,350 Beam Dec. 7, 1948 2,507,359 Weisz May 9, 1950 2,524,100 Dauvillier Oct. 3, 1950 FOREIGN PATENTS Number Country Date 534,710 Great Britain Mar. 14, 1941 

